Military land vehicles generally have a three-, four-, or five-point seat/shoulder belt or harness in one or more of the vehicle seats. Civilian vehicles generally have a three-point shoulder belt/lap belt harness. To minimize or reduce injury during a collision or rollover, both military and civilian vehicles have used seat harness pretensioners. In a typical pretensioner, sensors on the vehicle detect the acceleration and/or deceleration forces of a collision or rollover, causing the pretensioners to actuate. The pretensioners then tighten up the seat harness around the seated passenger or driver milliseconds before the actual full impact or rollover. The tightened harness holds the passenger securely into the seat, reducing impact and rebound injuries. These existing pretensioners can be relatively complicated designs because they rely on pyrotechnic devices or electric motors operating a belt retractor, each controlled by an electronic trigger.
Many military land vehicles are designed to resist the destructive forces of mines and improvised explosive devices (IEDs). These explosive devices create unique risks for military land vehicles. Detonation of a large explosive device can generate forces on the vehicle exceeding 100 g, causing the vehicle to accelerate violently upwardly. To reduce the forces on the vehicle crew during a detonation, some military vehicles, such as the Joint Light Tactical Vehicle (JLTV), Bradley Fighting Vehicle, and Mine Resistant Ambush Protected Vehicle (MRAP), have blast attenuating seats. Blast attenuating seats are often supported on shock absorbing materials or are mounted on a shock absorbing structure, such as a stroking device, that allows the seat to move downwardly during a blast.
Conventional harness pretensioning techniques are generally impractical with these types of blast attenuating seats, because the seats move relative to the vehicle body during the blast and the acceleration is largely vertical, rather than horizontal. In addition, conventional harness pretensioning systems typically use pyrotechnic devices, electric retractor motors, or electronic sensors, which can be complicated and costly.
In addition to land vehicles, passengers in helicopters are subjected to similar types of extreme forces during crashes or hard landings. Some helicopters also have seats mounted on a stroking device. The challenges presented in harness pretensioning with blast attenuation seats are also present in helicopter seats mounted on stroking devices.